The present invention relates to gas turbofan engines and, more particularly, to a double bypass, variable cycle gas turbofan engine wherein the bypass airflows may be controlled to permit elimination of an exhaust duct and an exhaust nozzle.
In recent years, much attention has been devoted to developing what have been generally referred to as "variable cycle" engines, a family of hybrid gas turbine engine which can operate with the high specific thrust characteristics of a low bypass ratio turbofan or turbojet as supersonic speeds and yet also exhibit the lower specific thrust, low noise and low fuel consumption characteristic of a high bypass ratio turbofan. The need for such variable cycle engines has arisen because of the need for an efficient multimission aircraft. Current aircraft having multimission requirements are powered by engines which are, of necessity, a compromise between the subsonic high bypass ratio turbofan and a supersonic augmented turbojet. As a result, what has emerged is a compromised low-to-moderate bypass ratio turbofan engine.
Several design approaches toward a truly variable cycle engine have been proposed. Such prior art systems have included various concepts employing retractable fans, variable area turbines, and variable pitch fans, as well as more complex techniques such as compound engines utilizing combinations of turbofan and turbojet engines in tandem or concentric flow relationship. In addition to a lack of flow flexibility, the weight associated with the "unused" portion of the engine during certain operating modes is an inherent disadvantage in aircraft applications.
One of the most attractive variable cycle engine concepts currently under consideration is a double bypass, variable cycle gas turbofan engine. The flow modulation potential is increased by splitting the fan into two sections, each in flow communication with a separate concentric bypass duct surrounding the core engine. It has been proposed to vary the bypass ratio (the ratio of the quantity of flow bypassing the core engine to that passing through it) by selectively mixing or separating one or more of the bypass duct streams and the core engine exhaust stream through various systems of valves and mixers.
However, one disadvantage to all such multiple bypass, variable cycle turbofan engines is that the number of exhaust nozzles has been equal to the number of flow ducts within the engine. Thus, the exhaust system for a double bypass, variable cycle engine required three separate ducts and nozzles to accommodate the three airstreams. Generally, as the number of ducts increases, weight increases. The present invention overcomes this disadvantage by reducing the number of exhaust nozzles and ducts required while still achieving the projected cycle variability of the double-type bypass, variable cycle engine.